Remove a rewrite rule that makes GHC 7.4.1 ill
Remove a rewrite rule that makes GHC 7.4.1 ill

The effect of the rule was to make isPrefixOf infloop; see [GHC bug
5783](http://hackage.haskell.org/trac/ghc/ticket/5783) for details.

This fixes gh-16.

alexliew/text,aloiscochard/text,text-utf8/text,bgamari/text,spencerjanssen/text,bos/text,ekmett/text,spencerjanssen/text,hvr/text,beni55/text,alexliew/text,spencerjanssen/text,23Skidoo/text,bos/text,23Skidoo/text,text-utf8/text,23Skidoo/text,beni55/text,spencerjanssen/text,bgamari/text,hvr/text,ekmett/text,aloiscochard/text,alexliew/text,aloiscochard/text,beni55/text,hvr/text,ekmett/text,bgamari/text,text-utf8/text,beni55/text,bos/text,bos/text,23Skidoo/text,ekmett/text,text-utf8/text,alexliew/text,aloiscochard/text,bgamari/text,hvr/text